Blowout preventers (BOPs) are commonly used substantially at or near the surface to control sub-surface pressures encountered when drilling subterranean wells. BOPs have been used for decades and are an important part of the well drilling system. BOP actuation forces for operating a ram block may include manual mechanisms and pneumatic or hydraulic pressure which may act upon a piston to close or open ram sealing elements. Hydraulic actuation may generally be preferred when the required closing forces are relatively high. The hydraulic actuation force may be applied to a cylinder containing a piston which in turn may act upon a shaft having a ram element attached to the near end of the shaft opposite the piston. In such installations, the closing force may be substantially equivalent to the effective cross-sectional area of the piston multiplied by the pressure of the hydraulic fluid.
In certain applications, it is desirable to apply an enhanced closing force to the rams, e.g., when using shear rams. To achieve the desired closing forces, a prior art solution has been to add a hydraulic booster to ram actuator assembly to increase the effective closing force for a given hydraulic actuation pressure. This prior art hydraulic booster is discussed subsequently. Alternatively, the hydraulic actuation pressure may be enhanced so as to increase the closing force without relying upon boosters. This latter technique may result in a preventer stack assembly which operates with hydraulic fluids at different pressures, thereby increasing system complexity. Preventer ram actuators may typically operate at hydraulic pressures of 1500 or 3000 psig. Where enhanced closing force is required, pressures may be increased to 5000 psig. Some components in the preventer stack may not be designed to handle the higher hydraulic pressures, thereby resulting in a preventer system with more than one independent pressure system. Increasing the hydraulic pressure upon the booster piston may undesirably increase the complexity and safety concerns involving the actuation system. All components in a preventer system may not be rated to the higher pressures, and more than one actuation system may have to be provided and operated to accommodate all components in the higher pressure system. Accordingly, the solution of raising the hydraulic pressure to the ram actuator is not favored, particularly when a substantial increase in the closing force is required.
A hydraulic booster may be placed in series with a main actuator piston. Typically the hydraulic booster provides a piston which has a larger cross-sectional area upon which the hydraulic pressure acts, thereby increasing the closing force. The booster piston may be attached to a far end of a booster guide rod and the near end of the booster guide rod may then act upon the high pressure side of the main actuator piston. The net closing force upon the primary piston shaft is accordingly increased by the mechanical force to the main actuator piston resulting from hydraulic pressure to the booster piston.
One problem with prior art booster pistons is the need for increased preventer height to accommodate the increased diameter of the booster piston. A short preventer stack height is typically preferred. U.S. Pat. No. 5,957,484 discloses a preferred technique to easily and reliably lock the door of a BOP actuating mechanism to the BOP body, with one benefit being a vertically short BOP. Booster pistons providing larger diameter pistons also require larger diameter housings which result in increased stack height. In addition, increased diameter piston housings may also require increased wall thickness, which, in combination with the increased housing circumference, may result in an excessive amount of weight to the BOP stack.
A second problem with prior art hydraulic boosters may be encountered where the booster guide rod acts upon a primary piston assembly having internal moving parts, such as an automatic internal locking system. Operation of the main actuator assembly may preclude the application of force, such as by a booster guide rod, to the center of the primary piston. For example, U.S. Pat. No. 5,575,452 discloses a blowout preventer ram actuator mechanism, wherein the primary piston includes an outer sleeve portion which supports an independently movable locking piston. The locking piston has tapered surfaces, and locking segments each engage one of a plurality of tapered locking rods fixed to the actuating mechanism housing. Due to the locking piston components which move independently of the primary piston, an axially centered boosting force may not be exerted directly against these internal moving parts without risking premature locking of the primary piston. Other BOP ram actuating mechanisms may similarly include centrally located parts which limit or preclude the use of a conventional booster piston.
An additional problem with prior art hydraulic boosters involves the increased number of seals between components. The addition of a hydraulic booster may expose additional sets of external seals, thereby increasing the risk of leaking hydraulic fluid to the environment.
The disadvantages of prior art are overcome by the present invention, and an improved ram actuating mechanism with hydraulic boost capability is hereafter disclosed. The improved ram actuating mechanism is relatively simple, is highly effective and reliably provides a mechanism to significantly enhance or boost the actuation force in a ram type preventer.